The use of metal-metal oxides in the electrical contact industry is well known. Such materials, particularly those of the silver-cadmium oxide family, have the advantage of substantially reducing the tendency for sticking in make-and-break contact applications. Further, electrical contacts fabricated from such materials exhibit good arc interruption characteristics, low contact resistance, and high resistance to electrical erosion. However, certain of such materials such as, for example, silver-cadmium oxide, which have superior functional qualities for electrical contact applications, do not lend themselves to low cost production techniques such as heading. Low formability of such materials tends to result in cracking during the heading operation and an accompanying loss of functional qualities. It is recognized that a high metal oxide content is desirable to cause dispersion hardening and to lend errosion resistance to contacts fabricated from such materials but the adverse effect of the metal oxide on formability has heretofore resulted in a limitation on the percentage of metal oxide possible in the final product.
Various techniques have been disclosed in the prior art for fabricating electrical contacts from such metal-metal oxide materials. For example, in British Pat. No. 1,397,319, there is disclosed a method for fabricating contact shapes by compacting silver-cadmium oxide powder under pressure to form a compacted body, sintering the compacted body and thereafter forming the compacted body into the desired contact shape. This technique has been found to result in three distinct manufacturing problems. First, due to the fact that many integral particles are being consolidated, there is an extremely large surface area which is quite difficult to keep free of contamination. Secondly, during consolidation of the particles, interfaces are formed having random directionality extending to the surface of the consolidated body where they act as stress raisers. Thirdly, the metal oxide particle size and distribution throughout the final product is very non-uniform and therefore difficult to control by virtue of varying oxidation paths inherent when variously sized integral particles are present.
Certain of the problems inherent in the technique noted above have been alleviated to some extent by other prior art techniques wherein a strip of silver-metallic alloy is manufactured by melting and casting bars of the alloy material and then rolling the bars to form strips of approximately the desired shape and thickness of the final product followed by internal oxidation of the strip and, where necessary, a second rolling operation to the final size. Still another technique involves extrusion of a pre-oxidized silver-metallic oxide material in the form of shot-grain or pellets. This technique, which is disclosed in U.S. Reissue Pat. No. Rei 27,075, generally comprises the steps of forming a silver-cadmium metallic shot, internally oxidizing the shot, compacting the oxidized shot, extruding the shot, and cold working the extruded shape to the final desired size.
Most recently, in U.S. Pat. Nos. 3,932,935 and 3,932,936, a method for manufacturing silver-metallic oxide materials from which electrical contacts can be prepared has been disclosed which comprises extrusion pressing an assembled plurality of plates or wires of silver-metallic oxide materials which had been previously prepared by either internal oxidation or powder metallurgical techniques. The extruded product thus produced is said to exhibit good formability when subsequently shaped into an electrical contact. While it is disclosed that the fibrous structure of the metallic oxide stratum that results from this technique will have a favorable effect on the subsequent handling of the extruded product when it is formed into an electrical contact, such stratum exists in only a single plane, thus resulting in less than a complete uniformity of metallic oxide distribution throughout the final product. This lack of complete uniformity can lead to the presence of stress raisers and consequently to an adverse affect on formability.
At present, several hundred different contact materials are presently manufactured in order to fill market requirements. Generally, a number of different functional qualities are needed in the contact material in order to provide the best performance results in a given application. For example, in the household circuit breaker market, silver-molybdenum-tungsten, silver-molybdenum, and silver-cadmium oxide materials are most widely used for the electrical contacts. On the other hand, in the appliance contact market, fine silver, silver-copper alloys, and silver-cadmium oxide materials are used extensively. In either case, each type of refactory material contributes a particular desirable quality to the contact material incorporating same. However, manufacturing limitations often cause compromises to be made and the optimum contact material composition cannot always be used.
Heretofore, the manufacture of composite contact materials has been accomplished by incorporating a desired additive material into the basic metal-metal oxide material in the form of a powder such as disclosed in British Pat. No. 1,397,319 discussed above, U.S. Pat. No. 3,158,469 and U. S. Pat. No. 3,827,883. Alternatively, the material has been incorporated by melting and casting it along with the basic materials into the form of an ingot prior to rolling the ingot to form a slab from which electrical contacts are then manufactured. This latter technique is disclosed in U.S. Pat. No. 3,694,197. It has also been suggested, in U.S. Pat. No. 3,821,848, that composite contact materials can be fabricated by metallurgically bonding a layer of a desired additive material directly to a metal-metal oxide electrical contact.
Each of the prior art techniques for manufacturing composite electrical contact materials suffers from the same drawbacks as those set forth above with respect to the manufacture of electrical contact materials in general. Further, the manufacture of composite electrical contact materials according to the prior art techniques does not result in as uniform a distribution of the additive material throughout the finished product as would be desirable. Again, this lack of complete uniformity can lead to the presence of stress raisers and to an adverse affect on formability, particularly in the case where the additional material is added prior to internal oxidation and becomes partially oxidized as well.
It is therefore an object of the present invention to provide a new method for the manufacture of improved electrical contact materials from metal-metal oxides which do not exhibit the above-noted drawbacks encountered with the presently known prior art techniques.
It is a further object of the present invention to provide improved electrical contact materials having a substantially symmetrical and uniform distribution of metallic oxide throughout and a plurality of substantially symmetrical, coaxial slip zones throughout the material which are substantially devoid of metal oxide, thus resulting in a greater degree of formability without danger of rupture as characteristic in the prior art.
It is still a further object of the present invention to provide a new method for the manufacture of improved electrical contact materials from metal-metal oxides which permits the inclusion of a higher metallic oxide content than possible according to the prior art techniques without sacrificing formability.
Still a further object of the present invention is to provide a new method for the manufacture of composite electrical contact materials which do not suffer from the drawbacks characteristic in the prior art.